Datasheet

207
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
• Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software response. If the I-bit in
SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT flag is set, the SCL low
period is stretched. The TWINT flag must be cleared by software by writing a logic one to it. Note that this flag is not
automatically cleared by hardware when executing the interrupt routine. Also note that clearing this flag starts the operation
of the TWI, so all accesses to the TWI address register (TWAR), TWI status register (TWSR), and TWI data register (TWDR)
must be complete before clearing this flag.
• Bit 6 – TWEA: TWI Enable Acknowledge Bit
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK pulse is
generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received.
2. A general call has been received, while the TWGCE bit in the TWAR is set.
3. A data byte has been received in master receiver or slave receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire serial bus temporarily. Address
recognition can then be resumed by writing the TWEA bit to one again.
• Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a master on the 2-wire serial bus. The TWI hardware
checks if the bus is available, and generates a START condition on the bus if it is free. However, if the bus is not free, the
TWI waits until a STOP condition is detected, and then generates a new START condition to claim the bus master status.
TWSTA must be cleared by software when the START condition has been transmitted.
• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in master mode will generate a STOP condition on the 2-wire serial bus. When the STOP
condition is executed on the bus, the TWSTO bit is cleared automatically. In slave mode, setting the TWSTO bit can be used
to recover from an error condition. This will not generate a STOP condition, but the TWI returns to a well-defined
unaddressed slave mode and releases the SCL and SDA lines to a high impedance state.
• Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI data register – TWDR when TWINT is low. This flag is cleared by
writing the TWDR register when TWINT is high.
• Bit 2 – TWEN: TWI Enable Bit
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the TWI takes control
over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters and spike filters. If this bit is written to
zero, the TWI is switched off and all TWI transmissions are terminated, regardless of any ongoing operation.
• Bit 1 – Reserved
This bit is a reserved bit and will always read as zero.
• Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for as long as the
TWINT flag is high.